I would recommend hitting up Steam and checking out their free game section. There are large variety of free games. Also, if you are a little more daring, you could hit f2p.com, there a lot of free games on there as well.

I would say if you're looking for a shooter, then go planetside 2. Large scale battles, and customization worth playing for. Hours of entertainment. If you're looking for more of an RPG then Lord of The Rings Online is one of the best F2P